The present disclosure relates to a storage device and a storage unit, which store information by means of a change in an electrical property of a storage layer including an ion source layer.
In general, NOR or NAND type flash memories are used as semiconductor nonvolatile memories for data storage. These types of semiconductor nonvolatile memories have achieved a large capacity by miniaturizing individual memory devices and drive transistors. However, it is pointed out that their miniaturizations are limited, because a high voltage is necessary to write or erase data, and the number of electrons injected into each floating gate is confined.
Currently, resistance change type memories, such as resistance random access memories (ReRAMs) or phase-change random access memories (PRAMs), are proposed as next generation nonvolatile memories that potentially exceed the limitation of the miniaturization (for example, see Japanese Unexamined Patent Application Publication No. 2006-196537 and Waser, et al., “Advanced Material” Volume 21, page 2932 (2009)). Such memories have a simple structure in which a resistance change layer is formed between two electrodes, and their writing and erasing operations are performed by means of the change in the resistance of the resistance change layer. One reason for the change in the resistance is considered that the atoms or ions travel with heat or an electric field to create a conductive path.
There is another technique for achieving a large capacity of memories which is called a multi-valued technique, in addition to the technique for miniaturizing the memories as described above. Specifically, this technique has achieved multivalued recording by enabling a single device to perform multiple-bit recording. If each device records 2 bit (4 value) or 3 bit (8 value) data, the total capacity is doubled or tripled.